|Title||A seismic survey in the Canadian Shield, I: refraction studies based on rockbursts at Kirkland Lake, Ontario|
|Author||Hodgson, J H|
|Source||Publications of the Dominion Observatory vol. 16, no. 5, 1953, 53 pages, https://doi.org/10.4095/8676|
|Publisher||Canada Department of Mines and Technical Surveys (Ottawa, Canada)|
|Media||paper; on-line; digital|
|Related||This publication is related to Hodgson, J H; (1953). A seismic
survey in the Canadian Shield, II: Refraction studies based on timed blasts, Publications of the Dominion Observatory vol. 16 no. 6|
|NTS||21 /NW; 21 /SW; 22 /SW; 31; 40 /NE; 41 /NE; 41 /SE|
|Area||Brentha; Dack; Dane; Earlton; Fabre; Hanbury Area; Hough Lake Area; Keewatin Area; Kirkland Lake Area; La Cave Area; Laniel Area; New Liskeard Area/que; Rolphton Area; Seven Falls Area; Shoomogen Area;
Sudbury Area; Tarzwell Area; Temiskaming Area; Ville-marie Area/ont|
|Subjects||geophysics; regional geology; tectonics; crustal studies; mohorovicic discontinuity; p waves; rockbursts; s waves; seismic interpretations; seismic velocities; seismic surveys; Algoman Granite; Canadian
Shield; Keweenawan Diabase; Lorrain Quartzite; Temiskaming Tuff; Phanerozoic; Paleozoic; Silurian; Ordovician; Precambrian|
|Illustrations||graphs; tables; photographs; location maps; schematic representations; plots; seismograms|
Natural Resources Canada Library - Ottawa (Earth Sciences)
|Released||1953 01 01; 2018 10 09|
|Abstract||Between 1938 and 1943 a number of large rockbursts at Kirkland Lake were recorded at permanent seismological observatories in eastern Canada and New England. Analysis of these records suggested that
rockbursts enjoy certain unique advantages as an energy source in crustal studies, since they are susceptible of precise location and timing and yet have about the same energy distribution as earthquakes. Immediately after the war steps were taken lo
set out a refraction profile for detailed studies of the crust.|
A seismograph was installed at Kirkland Lake to time the bursts at their source and 14 stations were occupied at distances varying from 8 to 174 km. The locations were occupied one or
two at a lime, the instruments being moved to new locations after bursts had been satisfactorily recorded. The stations were housed in portable prefabricated buildings especially designed for the purpose.
Several types of instruments were used
during the life of the project, those finally selected being of a type designed by Willmore for use in South African crustal studies. Particular attention was given to accurate timing and it is shown that the elapsed time of a single event has an
accuracy of the order of ± 0.06 seconds (p.e.).
The refraction profile obtained consists of the 14 field stations already mentioned plus 5 distant stations which recorded the earlier large bursts. First arrivals in the P and S groups suggest that
the crust is single layered, and the point at which P is first observed provides confirmation of this conclusion. In the analysis of secondary arrivals it becomes necessary to conclude that the crust, by lateral variation in rock types and by
variable thickness, provides several alternate paths for each ray, so that groups of phases are obtained rather than single distinct phases. Within these limits it is possible to account qualitatively for the secondary arrivals and lo conclude that
reflections are obtained from the base of the crust as the critical angle is approached and exceeded.
The records of the distant stations show a very large amplitude for about 10 sec. following the expected arrival time of the direct S waves. This
group appears to be identical with that called Lg by Ewing and Press.
P and S velocities in the crustal layer are 6.246 ± 0.015 and 3.544 ± 0.023 km/ sec., the uncertainties being probable errors. The P velocity below the Mohorovicic discontinuity
is 7.913 ± 0.125 km/ sec. if near-station data are used, and 8.176 ± 0.013 km/ sec. combining data of near and distant stations. This suggests an increase of velocity with with depth. Sn velocity, based on the records of the distant stations only, is
4.85 ± 0.10 km/ sec. The mean thickness of the crust, based on the P waves, is 35.4 ± 5.5 km., the uncertainty being the result of the uncertainties in the velocities, and not a true probable error. It is suggested that this uncertainty corresponds
to the actual variation in crustal thickness.